1. Field of the Invention
The present invention relates to a driving method for writing an image on a liquid crystal device displaying and recording an image by using a liquid crystal and a photoconductor, and a driving device for the liquid crystal device.
2. Description of the Related Art
There is increasing expectation of a rewritable marking technique as an alternative to paper as a hardcopy technique due to such reasons as protection of the global environment including forest resources and space saving.
The paper hardcopy has various advantages that the conventional electronic display devices do not have, i.e., (1) it attains reflective full color display with high brightness and contrast, can be easily read and has a high information displaying density, (2) it has such a structure that attains light weight, thinness and flexibility, and can be viewed in a comfortable position with wide room for choice of illumination conditions, (3) it can display and record information without power source owing to memory effect in displaying, and is free of eyestrain owing to display without flicker, and (4) it attains display at low cost, and easily provides list view with plural sheets simultaneously displayed, which facilitates comparison and browse of information.
Accordingly, there has been such a phenomenon that paperless environments are not facilitated in offices, and information displayed on an electronic display is printed on paper as a hardcopy for browsing. Therefore, a display medium as an alternative to paper should have the aforementioned various advantages inherent to paper in addition to rewritable capability for resource saving and reduction of wastes.
Various rewritable marking techniques having high convenience have been studied, and as one measure thereof, a liquid crystal device having a liquid crystal and a photoconductor accumulated on each other is receiving attention since information can be repeatedly recorded and deleted thereon, and various excellent characteristics are realized. As an example of the technique, the inventors have proposed a liquid crystal device in JP-A-11-237644. The liquid crystal device contains a liquid crystal layer and a photoconductor layer accumulated on each other, in which the liquid crystal layer is a self-maintaining liquid crystal composite containing a cholesteric liquid crystal and a transparent resin, and the photoconductor layer is an organic photoconductor layer, and has the following advantages. The liquid crystal device can form an image without simultaneous exposure over the entire surface of the liquid crystal device, and can reduce the size and cost of the writing apparatus. The liquid crystal device itself is thin and light, which facilitates handling. Accordingly, the liquid crystal device provides, as a total system, a rewritable display medium and a writing apparatus, which can be an alternative to a hardcopy.
The technique employed in the liquid crystal device of JP-A-11-237644 will be described briefly. A planar texture exhibited by a cholesteric liquid crystal (chiral nematic liquid crystal) separates light incident in parallel to the helical axis thereof into dextrorotatory light and levorotatory light, and causes such a selective reflection phenomenon that a circularly polarized component agreeing with the twist direction of the helix is reflected by Bragg reflection, and the remaining light component is transmitted. The center wavelength λ and the reflected wavelength range Δλ are expressed by the following equations:λ=n·p Δλ=Δn·p wherein p represents the helix pitch, n represents the average refractive index within the plane perpendicular to the helical axis, and Δn represents the birefringence within that plane. The light reflected by the cholesteric liquid crystal layer having a planar texture exhibits a bright color depending on the helix pitch.
A cholesteric liquid crystal having a positive dielectric anisotropy shows the following three states. That is, in a planar texture, the helical axis is perpendicular to the cell surface as shown in FIG. 14A, and the incident light is subjected to the aforementioned selective reflection phenomenon. In a focal conic texture, the helical axis is substantially in parallel to the cell surface as shown in FIG. 14B, and the incident light is transmitted with slightly forward scattering. In a homeotropic texture, the helical structure is unraveled to direct the liquid crystal director to the electric field direction as shown in FIG. 14C, and the incident light is substantially completely transmitted.
Among the three states, the planar texture and the focal conic texture can be bistability present under no electric field. Therefore, the phase state of a cholesteric liquid crystal is not determined unconditionally without the intensity of the electric field applied to the liquid crystal layer, and in the case where a planar texture appears as the initial state, the phase state is changed sequentially to a planar texture, a focal conic texture and a homeotropic texture in this order with increase of the intensity of the electric field, and in the case where a focal conic texture appears as the initial state, the phase state is changed sequentially to a focal conic texture and a homeotropic texture in this order with increase of the intensity of the electric field.
In the case where the intensity of the electric field applied to the liquid crystal layer is decreased suddenly to zero, the planar texture and the focal conic texture maintain the states as they are, and the homeotropic texture is changed to a planar texture.
Therefore, the cholesteric liquid crystal layer immediately after applied with a pulse signal shows switching behavior as shown in FIG. 15. That is, when the voltage of the pulse signal applied is Vfh or higher, a selective reflection state where a homeotropic texture is changed to a planar texture appears. When the voltage is from Vpf to Vfh, a transmission state with a focal conic texture appears. When the voltage is Vpf or lower, the state before applying the pulse signal is continued, i.e., a selective reflection state with a planar texture or a transmission state with a focal conic texture appears.
In FIG. 15, the ordinate shows the normalized reflectivity, which is obtained by normalizing the reflectivity with the maximum reflectivity as 100 and the minimum reflectivity as 0. A transition state appears among the planar texture, the focal conic texture and the homeotropic texture, and therefore, it is determined that the case where the normalized reflectivity is 50 or more is designated as the selective reflection state, the case where the normalized reflectivity is less than 50 is designated as the transmission state, the threshold voltage of phase transition from the planar texture to the focal conic texture is designated as Vph, and the threshold voltage of phase transition from the focal conic texture to the homeotropic texture is designated as Vfh.
Particularly, in the PNLC (polymer network liquid crystal) structure containing a network resin in a continuous phase of a cholesteric liquid crystal, and the PDLC (polymer dispersed liquid crystal) structure containing a cholesteric liquid crystal dispersed as droplets in a polymer skeleton (including those microencapsulated), the bistability of a planar texture and a focal conic texture under no electric field is increased with interference at an interface between the cholesteric liquid crystal and the polymer (anchoring effect), whereby the state immediately after applying a pulse signal can be maintained for a long period of time.
In the liquid crystal device using the technique, (A) the selective reflection state with a planar texture and (B) the transmission state with a focal conic texture are switched by utilizing the bistability phenomenon of the cholesteric liquid crystal, so as to realize black/white monochrome display having a memory effect under no electric field or color display having a memory effect under no electric field.
In the liquid crystal device using the technique, further more, the self-maintaining liquid crystal composite and the organic photoconductor can be formed and accumulated by such a measure as coating a coating composition or laminating, and therefore, the liquid crystal device can be easily produced at low cost. The self-maintaining liquid crystal composite and the organic photoconductor can attain such a resolution that is required for a hardcopy and can attain sufficiently high resolution of the liquid crystal device.
The liquid crystal device using the technique can form an image over the entire surface thereof without exposure, and thus an image can be written therein by scanning the surface of the liquid crystal device by using a scanning exposure device, such as a laser exposure device and a light-emitting diode array.
FIG. 16 is a schematic cross sectional view showing the liquid crystal device using the technique where an image is written therein with a scanning exposure device. As shown in FIG. 16, the liquid crystal device using the technique contains a display layer as a liquid crystal layer, an OPC layer as a photoconductor layer and, for example, a light absorbing layer, which are held between a pair of transparent electrode substrates. After resetting the entire surface of the display layer to a planar texture, the surface thereof on the side of the OPC layer is exposed imagewise by scanning with an exposure device, such as a line head or a beam scanner, under application of a prescribed bias voltage between the transparent electrodes, whereby a desired image can be written therein.
As having been described, the liquid crystal layer upon writing forms a desired image by producing contrast between a part that undergoes transition from a planar texture to a focal conic texture and a part that does not undergo the phase change, depending on the presence or absence of exposure. The transition from a planar texture to a focal conic texture requires a certain period of time. Specifically, the liquid crystal layer completes the phase change over several hundreds milliseconds (about 200 ms or more), and thus a writing time of several hundreds milliseconds is consumed per one scanning line (or one pixel). As a result, a prolonged period of time is required for rewriting the entire surface of the liquid crystal device in total. Consequently, the aforementioned technique is still insufficient in practicality on writing.
Under the circumstances, the invention provides such a driving method of a liquid crystal device and a driving device of a liquid crystal device that have the following features. The liquid crystal device can form an image without simultaneous exposure over the entire surface of the liquid crystal device, and can reduce the size and cost of the writing apparatus. The liquid crystal device itself is thin and light, which facilitates handling. Accordingly, the liquid crystal device provides, as a total system, a rewritable display medium and a writing apparatus, which can be an alternative to a hardcopy. More specifically, the liquid crystal display device has a self-maintaining liquid crystal composite containing a cholesteric liquid crystal and a transparent resin as a liquid crystal layer, and an organic photoconductor layer as a photoconductor layer, which are accumulated to each other, and upon writing an image in the liquid crystal device through exposure with a scanning exposure device, an image can be written in a short writing time to impart high practicality to the driving method and driving device.